Since the cost of a well increases with the bore diameter, it is desirable for pump assemblies to be of small external diameter and high rotational speed in order to secure the same pumping performance as with a pump of a lower rotational speed but of larger external diameter. Such a smaller diameter high speed pump assembly is subject to greater stress on the pump motor, with the attendant problem of dissipating resistance losses occurring in the pump motor.
Frequency changers comprising an operating electronic system are frequently used for augmenting the speed of rotation of pump motors. If such a frequency changer is an integrated component of the pump motor, the supplementary problem arises that heat losses of said electronic system need to be dissipated.